//给定一个二叉树，返回其节点值自底向上的层次遍历。 （即按从叶子节点所在层到根节点所在的层，逐层从左向右遍历） 
//
// 例如： 
//给定二叉树 [3,9,20,null,null,15,7], 
//
//     3
//   / \
//  9  20
//    /  \
//   15   7
// 
//
// 返回其自底向上的层次遍历为： 
//
// [
//  [15,7],
//  [9,20],
//  [3]
//]
// 
// Related Topics 树 广度优先搜索 
// 👍 358 👎 0


//leetcode submit region begin(Prohibit modification and deletion)

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.Stack;

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        if (null == root) {
            return new ArrayList<>();
        }
        LinkedList<List<Integer>> result = new LinkedList<>();
        Stack<TreeNode> rightToLeft = new Stack<>();
        Stack<TreeNode> leftToRight = new Stack<TreeNode>();
        rightToLeft.push(root);
        while (!rightToLeft.isEmpty()){
            while (!rightToLeft.isEmpty()){
                leftToRight.push(rightToLeft.pop());
            }
            List<Integer> list = new ArrayList<>();
            while (!leftToRight.isEmpty()){
                TreeNode pop = leftToRight.pop();
                list.add(pop.val);
                if (null != pop.left) {
                    rightToLeft.push(pop.left);
                }
                if (null != pop.right) {
                    rightToLeft.push(pop.right);
                }
            }
            result.addFirst(list);
        }
        return result;
    }
}
//leetcode submit region end(Prohibit modification and deletion)
